The present invention relates to adjustable blowdown safety valves for use with gases on a pressurized system, such as pressure vessel or a flow line. Adjustable blowdown safety valves are commonly used on pressure vessels or pipelines to relieve temporary pressure surges in excess of the safe pressure the vessel or pipeline can withstand. Adjustable blowdown safety valves have the advantage of responding very quickly to pressure changes in pressurized systems to which they are attached. Adjustable blowdown safety valves move to a fully open position almost immediately after the pressure within the system rises above a user-determined set pressure. This allows excess fluid pressure to escape quickly. Then, when sufficient pressure has escaped, adjustable blowdown safety valves quickly move back to a closed position. For an example of an adjustable blowdown safety valve, see U.S. Pat. No. 3,572,372, which is herein incorporated by reference in its entirety.
The term “blowdown” refers to the percentage difference between the user determined set pressure and the pressure in the pressure vessel or flow line when the safety valve snaps closed. For example, if a user sets the set pressure at 100 psi, the safety valve stays open as gas escapes out of the safety valve until the pressure in the pressure vessel or flow line is 90 psi, then this safety valve has a blowdown value of 10%. Safety valves having a means to adjust the blowdown value while maintaining a constant set pressure are referred to as adjustable blowdown safety valves. Adjustable blowdown safety valves are desirable because they typically have low blowdown values of 5-15%, which minimizes the amount of gas that is lost from the pressurized system into the atmosphere during venting, thereby addressing environmental concerns. Safety valves not having a means to adjust the blowdown value while maintaining a constant set pressure are referred to as fixed blowdown safety valves. Fixed blowdown safety valves typically have blowdown values of 30-50%.
Existing adjustable blowdown safety valves do, however, have some problems. One problem with existing adjustable blowdown safety valves is that they tend to utilize a design where disk member lift is susceptible to change because of debris buildup on the lift stop. See, for example, the adjustable blowdown safety valves described in U.S. Pat. No. 3,411,530 (hereinafter the '530 patent), which is herein incorporated by reference in its entirety, and U.S. Pat. No. 5,341,838 (hereinafter the '838 patent), which is herein incorporated by reference in its entirety. In the '530 and '838 patents, as the disk member lifts off the inlet, fluid is directed around the outer diameter of the disk member towards the backside of the disk member. In the '530 patent, disk member lift is limited by a cage member. In the '838 patent, disk member lift is limited by an internal shoulder on the bonnet. Any build-up of foreign particles from contaminated fluids in designs utilizing these types of disk member stops can shorten disk member travel or interfere with disk member movement, which makes valve performance less reliable.
Another problem with existing adjustable blowdown safety valves is that they tend to utilize a design where the disk member outer diameter travels directly over areas in the direct path of flow from the inlet. In the '530 and '838 patents, the lower section of the outer diameter of the disk member overlaps the inlet throughout valve cycling. In the '530 patent, as the disk member lifts off the inlet, fluid is directed towards the inner diameter of the cage. In the '838 patent, as the disk member lifts off the inlet, fluid is directed towards the inner diameter of the bonnet. Any build-up of foreign particles from contaminated fluids coming from the inlet on the close fit surfaces can interfere with disk member movement, which makes valve performance less predictable.
In addition to the above-mentioned problems, adjustable blowdown safety valves can vary in quality in a number of ways. Different adjustable blowdown safety valves vary in their ability to attain high coefficients of flow, their performance reliability, their durability, their cost to manufacture, and their ease of use.